In modern vehicles there is an increasing tendency to provide functions which require an electrical load to remain supplied with power and functional at all times, or at least at all times when in a state of surveillance, even when the user of the vehicle has closed and immobilised the vehicle. This is especially so as regards systems for giving access to the vehicle, which enable the latter to be opened through a telecontrol system in which a radio frequency signal embodying an authorised code is transmitted to a receiver on board the vehicle. It is also the case as regards alarm systems using interrogation signals, which lead to an alarm being given if a predetermined event is detected, such as unauthorised intrusion or breaking of the glass of the cabin of the vehicle.
These systems are permanently connected to the vehicle battery, and sometimes to emergency power supplies, in particular with a view to giving protection against malicious damage or the occurrence of faults. Because of this it is desirable to arrange that these systems shall consume as little current as possible.
In addition, such systems must comprise electronic circuitry of very high performance, because the signals received by the system are generally of very low amplitude.
In French patent application No. 93 03191, filed on Mar. 19, 1993, the present Applicant has already proposed a circuit which is constructed around an intermediate frequency amplifier having at least two amplifying stages connected in series to the electrical power supply, with its input terminals being connected in cascade. Such an arrangement is perfectly satisfactory in the performance range envisaged, particularly in regard to electrical power consumption. However, in this circuitry, dynamic coupling of the two stages is obtained through a pair of transformers. These transformers must be matched to the working frequency. In addition they tend to be unduly sensitive to temperature, and they occupy a significant amount of space. Besides all this, the transformers are sensitive to electromagnetic fields in the environment, and in modern vehicles these fields are tending to become stronger.